The present invention relates generally to the field of optical signal processing apparatus, and more specifically to a spatial light modulator of simplified construction and improved performance.
Two-dimensional spatial light modulators are devices which allow control of an optical wavefront for processing or imaging operations. These devices, often referred to as light valves in the literature, have potential for application in large screen display systems as well as in optical data processing systems, including missile guidance and robotic vision systems. Listed below are several articles which describe their construction and operation.
1. "A Fast Silicon Photoconductor-Based Liquid Crystal Light Valve", P. O. Braatz, K. Chow, U. Efron, J. Grinberg and M. J. Little, IEEE International Electron Devices Meeting, pp 540-543, 1979. 2. "Oblique-cut LiN.sub.b O.sub.3 Microchannel Spatial Light Modulator", C. Warde and J. I. Thakara, Optics Letters, Vol. 7, No. 7, July 1982.
3. "A First-Order Model of a Photo-Activated Liquid Crystal Light Valve", J. D. Michaelson, SPIE Vol. 218, Devices and Systems For Optical Signal Processing, 1980.
4. "LiNbO.sub.3 and LiTaO.sub.3 Microchannel Spatial Light Modulators", C. Warde, A. M. Weiss and A. D. Fisher, SPIE Vol. 218, Devices and Systems for Optical Signal Processing, 1980.
5. "Silicon Liquid Crystal Light Valves: Status and Issues", U. Efron, P. O. Braatz, M. J. Little, R. N. Schwartz and J. Grinberg, Proc. SPIE Vol. 388, January 1983.
Basically, spatial light modulators generally comprise a photosensitive semiconductor substrate or wafer, a light blocking layer, a dielectric mirror and an electro-optic crystal (which may be a liquid crystal), arranged in a sandwich-like composite structure, and having a voltage applied thereacross. A control (write) illumination impinges on the face of the photosensitive semiconductor while an output (read) illumination makes a double pass through the electro-optic crystal.
The photosensitive semiconductor responds to intensity variations in the control illumination impinging thereon. In the dark, most of the voltage applied across the composite structure appears across the reverse-biased photodiode. The write beam, however, excites carriers in the silicon, which are driven by the internal field to the Si-electro-optic crystal interface. The voltage across the silicon decreases, while the voltage across the electro-optic crystal increases. The read illumination passes through the electro-optic crystal, is reflected off of the dielectric mirror, and again passes through the electro-optic crystal before emerging from the device. Since the diffraction efficiency of the electro-optic crystal is a function of the voltage applied thereacross, (which is a function of the intensity of the write illumination), optical control of the output (read) illumination is achieved.
One of the problems encountered in the practical implementation of such spatial light modulators is that of lateral charge transfer in the semiconductor device. By this is meant that the charge formed in the photosensitive semiconductor spreads at the surface of the semiconductor nearest the electro-optic crystal, resulting in poor spatial resolution.
One solution to the lateral charge transfer problem is mentioned by P. O. Braatz et al in the article entitled "A Fast Silicon Photoconductor-Based Liquid Crystal Light Valve", supra. It is stated therein that the spatial resolution of the input image across the silicon can be retained by means of a boron-implanted p-grid at the Si/SiO.sub.2 interface. The boron implanted p-grid acts to focus the incoming electrons into the resolution cell defined by it, as well as to form charge buckets of the electrons already residing at the Si/SiO.sub.2 interface. The overall result is to prevent lateral spill-over and consequent smearing of the charge pattern.
The aforementioned solution to the lateral charge transfer problem, however, is not without its own problems. The fabrication of low leakage p-n junctions in high resistivety Si is a difficult art, and the high temperature processing involved often leaves the Si surface distorted, degrading the performance of the device.